Eggshells serve as multifunctional shields for successful embryogenesis, such as protection, moisture control and thermal regulation. Unlike calcareous avian eggshells which are brittle and hard, reptilians have leathery eggshells that are tough and flexible. In this study, we investigate structure and mechanical properties of eggshells of Taiwan cobra snake (Naja atra) which composed mainly of keratin. Mechanical tests showed that snake eggshells are highly extensible and reversible. The exceptional deformability (110-230% tensile strain) and toughness of snake eggshells are contributed by the wavy and random arrangement of keratin and collagen fibers. Multi-scale toughening mechanisms of snake eggshells were observed and elucidated, including crack deflection and twisting, fibers reorientation, sliding and bridging, inter-laminar shear effect, and the - phase transition of keratin. Inspirations from the structural and mechanical designs of snake eggshells may lead to the synthesis of tough, extensible, lightweight composites which can be applied in various fields.

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